Advanced resists usually employ a technique called chemical amplification in which an acid generated by photolysis catalyzes a solubility switch from alkali insoluble to alkali soluble by removal of an acid sensitive group protecting an alkali solubilizing moiety. Polymers frequently used in this type of photosensitive composition include acetals derived from reaction of vinyl ethers with a polymer containing hydroxystyrene units. Chemically amplified resists based on acetal protected polyhydroxystyrene, such as found in U.S. Pat. Nos. 5,928,818, 5,834,531, and 5,558,976, which are incorporated herein by reference, are well known. Preferred characteristics and often advantages over other chemically amplified systems include lower temperature in processing conditions, and lower sensitivity to bake temperature variations.
As the semiconductor industry requires smaller and smaller features, the photoresists employed in the manufacture of semiconductor devices require improved resolution. As the required resolution becomes smaller, previously minor problems become more important to solve. Two such problems are a dependence of performance on film thickness and line collapse. As resolution requires increase, it is also more important to obtain as high a depth of focus as possible to help maintain processing latitude. The objective of this invention is to provide a photosensitive composition with high resolution, excellent photospeed, and excellent DOF at the same time as decreasing line collapse and film thickness dependence.